Variable displacement pump with electric control of displacement regulation and method of regulating pump displacement

ABSTRACT

A rotary positive displacement pump for fluids, in particular, for the lubrication of a motor vehicle engine ( 61 ), has a displacement that can be regulated through the rotation of a stator ring ( 42 ) having an eccentric cavity ( 43 ) in which the rotor ( 15 ) of the pump ( 1 ) rotates. The stator ring ( 42 ) is housed within an in eccentric cavity ( 13 ) of an external ring ( 12 ). A rotor actuator ( 50 ), controlled by the electronic control unit of the motor vehicle, causes a synchronous rotation by an equal amount in opposite directions of both rings. A method of regulating the displacement of the pump ( 1 ) and lubrication system for a motor vehicle engine in which the pump is used are also provided.

TECHNICAL FIELD

The present invention relates to variable displacement pumps, and moreparticularly it concerns a rotary positive displacement pump of the kindin which the displacement variation is obtained by means of the rotationof an eccentric ring (stator ring).

The invention also concerns a method of regulating the displacement ofsuch a pump.

Preferably, but not exclusively, the present invention is employed in apump for the lubrication oil of a motor vehicle engine.

PRIOR ART

It is known that, in pumps for making lubricating oil under pressurecirculate in motor vehicle engines, the capacity, and hence the oildelivery rate, depends on the rotation speed of the engine. Hence, thepumps are designed so as to provide a sufficient delivery rate at lowspeeds, in order to ensure lubrication also under such conditions. Ifthe pump has a fixed geometry, at high rotation speed the delivery rateexceeds the necessary rate, so that part of the delivered flow is to bedischarged in order to limit the delivery rate and the pressure. Ofcourse, the discharged oil volume has already been compressed, wherebyhigh power absorption occurs, with a consequent higher fuel consumptionand a greater stress of the components due to the high pressuresconstantly generated in the pump.

In order to obviate this drawback, it is known to equip the pumps withsystems allowing a delivery rate regulation at the different operatingconditions of the vehicle, in particular through displacementregulation. Different solutions are known to this aim, which arespecific for the particular kind of pumping elements (external orinternal gears, vanes . . . ).

A system often used in rotary pumps employs a stator ring with aninternal cavity, eccentric relative to the external surface, insidewhich the rotor, in particular a vane rotor, rotates, the rotor beingeccentric with respect to the cavity under operating conditions of thepump. By making the stator ring rotate by a given angle, the relativeeccentricity between the rotor and the cavity, and hence thedisplacement, is made to vary between a maximum value and a minimumvalue, substantially tending to zero (stall operating condition). Asuitably calibrated opposing resilient member allows the rotation when apredetermined delivery rate is attained and makes the pump substantiallydeliver such a predetermined delivery rate under steady stateconditions. An example is disclosed in U.S. Pat. No. 2,685,842.

Pumps with a pair of stator rings are also known, where displacement isvaried by rotating the rings relative to each other in oppositedirections. An example is disclosed in U.S. Pat. No. 4,406,599.

The evolution of such pumps and the diffusion of electronics in motorvehicle engines have lead to displacement regulation systems controlledby the electronic control unit of the vehicle depending on the oilpressure, preferably detected downstream the filter, and possibly onother operating parameters of the engine. Generally, such systems areelectro-hydraulic systems, where the control unit controls electricallyoperated valves that, in turn, control hydraulic actuators acting on thestator ring. For instance, US 2011/0209682 discloses a system in which acontrol module of the pump, being part of the electronic control unit,controls through an electrically operated valve the flow of pressurisedoil towards either of two chambers, which apply the oil pressure to thestator ring. Application of the pressure of either chamber correspondsto two different pressure/delivery rate conditions of the pump.

Generally, the provision of the hydraulic actuators makeselectro-hydraulic systems complex and expensive. Moreover, when theengine is off, it is impossible to modify the displacement presetting,since no control pressure is available.

It is an object of the present invention to provide a variabledisplacement pump, and a method of regulating the displacement of such apump, which obviate the drawbacks of the prior art.

DESCRIPTION OF THE INVENTION

According to the invention, this is obtained in that the pump includesan electromagnetic rotary actuator, integrated into or coupled with thepump, which is driven by an electronic system detecting operatingconditions of the pump and is arranged to transmit the rotary motion tothe stator ring.

Advantageously, the stator ring is housed within an eccentric cavity ofan external ring, and the rotary actuator is arranged to simultaneouslytransmit the rotary motion to both rings, in such a way as to cause asynchronous rotation thereof by an equal amount in opposite directions.

The invention also implements a method of regulating the displacement ofa rotary positive displacement pump by means of the rotation of aneccentric stator ring inside which the rotor rotates, the methodcomprising the steps of:

-   -   providing an electromagnetic rotary actuator integrated into or        coupled with the pump;    -   supplying the actuator with rotation commands corresponding to a        desired rotation of the stator ring.

Preferably, the method further comprises the steps of:

-   -   providing an external ring having an eccentric cavity within        which the stator ring is housed; and    -   making both rings rotate by a same angle at the same time and in        opposite directions.

According to a further aspect of the invention, a lubrication system fora motor vehicle engine is also provided, in which the adjustabledisplacement pump and the method of regulating the displacement setforth above are employed.

BRIEF DESCRIPTION OF THE FIGURES

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments, given by way ofnon limiting examples with reference to the accompanying drawings, inwhich:

FIG. 1 is a plan view of a first embodiment of the pump according to theinvention, from which the cover and the regulation actuator have beenremoved, in the minimum displacement position;

FIG. 2 is a view similar to FIG. 1, in the maximum displacementposition;

FIG. 3 is a plan view similar to FIG. 2, showing the delivery rateregulation mechanism integrated in the cover;

FIG. 4 is a cross-sectional view of the pump taken according to a planepassing through line Y-Y in FIG. 3;

FIGS. 5 and 6 are views similar to FIGS. 1 and 2, relating to a secondembodiment of the pump according to the invention; and

FIG. 7 is a principle block diagram of the displacement regulatingcircuit.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIGS. 1 to 4, a pump 1 according to the invention, moreparticularly a vane pump, includes a body 10 having a cavity 11 withsubstantially circular cross-section in which a first movable ring 12(external ring) is located. The ring in turn has an axial cavity 13,also with substantially circular cross-section, eccentrically arrangedrelative to cavity 11. A second movable ring 42 (stator ring) is locatedin cavity 13, which ring in turn has an axial cavity 43, also withsubstantially circular cross-section, eccentrically arranged relative tocavity 13 and having a centre O′. Rings 12 and 42 are arranged to rotatein mutually opposite directions by a certain angle in order to vary thepump displacement, as it will be disclosed below. Advantageously,cavities 13, 43 have the same eccentricities. In the exampleillustrated, cavity 11 is blind and is closed at one end by a cover 14(FIG. 4), also closing the corresponding ends of cavities 13, 43.

Cavity 43 in turn houses a rotor 15, rigidly connected to a drivingshaft 15 a making it rotate about a centre O, for instance in clockwisedirection, as shown by arrow F. Cavity 43 thus forms the pumpingchamber. In a minimum displacement position (shown in FIG. 1), rotor 15and cavity 43 are coaxial or substantially coaxial, whereas, in amaximum displacement position (shown in FIG. 2), centres O and O′ arelocated on the same axis X-X and are mutually spaced apart, and rotor 15is substantially tangent to side surface 43 a of cavity 43. In thepresent description, the term “coaxial or substantially coaxial” is usedto denote a minimum distance, tending to 0, between centres O and O′.

Advantageously, eccentric rings 12 and 42 are mounted in such a mannerthat, in the minimum displacement position, external ring 12 is orientedso that its minimum radial thickness is located at the top in the Figureand internal ring 42 is oriented so that its minimum radial thickness islocated at the bottom in the Figure. Otherwise stated, theeccentricities of the respective cavities 13, 43 are offset by 180°.Preferably, cavities 13, 43 have the same eccentricity relative to theexternal surface of the respective ring.

Rotor 15 has a set of vanes 16, radially slidable in respective radialslots. At an outer end, vanes 16 are at a minimum distance from sidesurface 43 a of cavity 43, whereas at their inner end they rest onguiding or centring rings 17, mounted at the axial ends of rotor 15 andarranged to maintain the minimum distance between vanes 16 and surface43 a under any condition of eccentricity. Also centring rings 17 will becoaxial or substantially coaxial with rotor 15 in the minimumdisplacement position.

A suction chamber 18, communicating with a suction duct 20, and adelivery chamber 19, communicating with a delivery duct 21, are definedat the bottom of body 10 between rotor 15 and surface 43 a. Suchchambers are substantially symmetrical with respect to a plane passingthrough axis X-X and have phasings that are ideal for the maximumvolumetric efficiency, as it is clearly apparent for the skilled in theart. It is to be appreciated that, should the rotor rotate incounterclockwise direction, the functions of such chambers, and hence ofthe respective ducts, would be mutually exchanged

In order to control the rotation of rings 12, 42, toothed sectors 51, 52are formed on their facing surfaces and are preferably positioned at thebase of suitable stator cavities 11 a, 11 b formed in rings 12, 42. Atoothed wheel 53 having a shaft 54 rigidly connected to an actuator 50(FIG. 4) driving it into rotation is interposed between toothed sectors51, 52 located in said stator cavities 11 a, 11 b. Thus, rings 12, 42rotate in opposite directions and are synchronous with each other.

Preferably, actuator 50 is an electromagnetic actuator. It may be arotary actuator, e.g. a step-by-step micromotor integrated into pump 1or coupled therewith (e.g. interfaced through the partition wallseparating the inside from the outside of the engine sump), or alinearly moving actuator coupled with a suitable escapement ratchet gearin order to convert the actuator motion into a rotary motion.

Actuator 50 is controlled by the electronic control unit of the vehicle,which manages the displacement variation in closed loop (e.g. withfeedback), by increasing or reducing the displacement depending on therequirements of the thermal engine and the accessories thereof. Thevariation is independent of the pressures upstream and downstream theoil filter.

Shaft 54 is guided within a support 40 formed in cover 14 or in body 10.Toothed sectors 51, 52, while rotating, develop according to a profiledefined by the involute of the teeth of wheel 53, which, on thecontrary, rotates about its stationary axis. If the eccentricities arethe same, the relative rotation of the rings causes a translation ofcentre O′ of pumping chamber 43 along axis X-X. This makes the geometryof pumping chamber 43 perfectly symmetric in all displacementconditions, and makes the ratio between the rotation of toothed wheel 53and the displacement variation because of the translation of axis ofchamber 43 constant.

In the illustrated embodiment, wheel 53 cooperates with a member 34opposing the rotation of rings 12, 42, in particular a flat spiralspring, preloaded so as to prevent the rotation of the rings as long asthe torque applied by actuator 50 is lower than a predeterminedthreshold. Spiral spring 34 is located in a casing 33 that, in theillustrated exemplary embodiment, is fastened to cover 14. The inner endportion of spring 34 is so shaped as to be coupled with the end portionof shaft 54 of wheel 53, whereas the outer end portion is locked to theinternal wall of casing 33. The latter may be rotated, for instance byusing a dynamometric key, in order to adjust the preloading of spring34. A ring nut 55 allows blocking casing 33 in the desired calibrationposition, independently of the constructional tolerances of the wholemechanism. A sealing gasket 56 is moreover provided between casing 33and cover 14 in order to isolate the internal chamber of the same casingfrom the outside. A drain puts such a chamber in communication withsuction chamber 18, for the aims that will be disclosed below.

It is to be appreciated that, during the regulation rotation, spiralspring 34, thanks to the negligible variation of the twisting torque andto the transmission ratio of the gear mechanism, will undergo negligiblevariations of its torque opposing the hydraulic torque. In the preferredembodiment in which actuator 50 is a step-by-step motor, spring 34 maycontribute to make the magnetic resistance torque between subsequentsteps sufficient to maintain the position of rings 12, 42 when the motorin not excited (energy saving). Moreover, due to a diametricallyopposite effect, spring 34 could contribute to maintaining a maximumdisplacement upon the occurrence of an electric failure.

Rings 12 and 42, as well as centring rings 17, rotor 15 and wheel 53,are preferably formed by moulding and/or metal powder sintering, withpossible finishing operations on some limited areas, according to thedictates of the art. More particularly, axial thicknesses will undergofinishing. Body 10 and cover 14 can be formed by moulding either analuminium alloy or a thermoplastic and/or thermosetting resin.Advantageously, spring 34 may be made of a bimetallic material, so thatits characteristic may change depending on the operation temperature.

A second embodiment of the pump according to the invention, denoted 101,is shown in FIGS. 5 and 6. Elements that are functionally identical tothose already disclosed with reference to FIGS. 1 to 4 are preferablydenoted by the same reference numerals, increased by 100. Pump 101differs from pump 1 in that external ring 12 is lacking and thereforeactuator 150 acts through wheel 153 onto stator ring 142 alone, which isformed internally of body 110 with substantially circular cross-section.

In accordance with such an embodiment, as shown in FIGS. 5 and 6, statorring 142 preferably comprises a stator cavity 111 in which both toothedsector 152 and toothed wheel 153 are arranged.

More particularly, toothed sector 152 is located at the base of statorcavity 111 and the toothed wheel is preferably wholly included betweentoothed sector 152 and body 110 with substantially circularcross-section.

Thanks to such a structure, in accordance with the second embodiment,the arrangement of toothed sector 152 and toothed wheel 153 allowsminimising the size of pump 101.

Moreover, rotor 115 rotates in counterclockwise direction (arrow F′).With such an arrangement, the translation of centre O′ of chamber 143takes place along a non-rectilinear trajectory. Apart from thoseaspects, the structure is identical to that of pump 1 ad it is notnecessary to describe it again.

FIG. 7 shows a principle block diagram of the regulation of thedisplacement of pumps 1, 101. Dashed line denotes the mechanical driveof the pump by actuator 50 and hence corresponds to toothed wheels 53,153 of the previous Figures. Dotted and dashed line 60 denotes thelubrication circuit which conveys oil from pump 1 to the engine and thevarious accessories, denoted in the whole 61. Reference numeral 62denotes the electronic control unit of the vehicle, which receivessignals from detectors denoted in the whole 63 and controls actuator 50,possibly through a digital-to-analogue converter, not shown. Solid linesdenote the paths of the electric signals incoming into/outgoing fromcontrol unit 62, and dotted lines denote the detection of the operatingparameters of engine 61, pump 1, lubrication circuit 60 and possiblyactuator 50 by detectors 63. The parameters on which regulation of thedelivery rate of the pump for lubrication of a motor vehicle engine maydepend are well known to the skilled in the art and are not of interestfor the invention. A more detailed description can be found in US2011/0209682.

The operation of the pump described is as follows.

Considering first pump 1, under rest conditions, the pump is in themaximum displacement condition shown in FIG. 2. As said, centre ofrotation O of rotor 15 is offset relative to centre O′ of cavity 43 ofeccentric ring 42 and rotor 15 is located close to wall 43 a of thecavity. When pump 1 is started, the clockwise rotation of rotor 15 willgive rise to an oil flow through chamber 19 and the associated deliveryduct 21 and, at the same time, an equal volume of oil will be suckedfrom chamber 18 and the associated suction duct 20. As the rotationspeed and the flow rate increase, the lubrication system of the engine,by opposing an increasing resistance to the flow, will make pressureincrease.

The delivery pressure or the pressure downstream the oil filter aredetected by the suitable detectors 63 and communicated to control unit62, which will make actuator 50 rotate. The actuator will in turngenerate a rotation torque that, through wheel 53 and once thecalibration value of counteracting spring 34 has been attained, willmake rings 12, 42 rotate by the same angle in opposite directions. If,as it has been assumed, the eccentricities of cavities 13, 43 relativeto the external surfaces of the respective rings are the same, therotation of ring 42 will cause a rectilinear translation of centre O′towards the right, proportional to the amount of the rotation, therebyproportionally reducing the eccentricity between rotor 15 and cavity 43,and consequently the pump displacement, and stabilising the pressure atthe calibration value. As parameters such as the speed, thefluidity/temperature of the fluid, the engine “permeability” (intendedas the amount of oil used by the engine) and so on, detected bydetectors 63, change, such a pressure will be maintained and controlledthrough the variation of the eccentricity and hence of the displacement.

When, as a function of the different operating parameters of the engine,it is desired to operate at a lower pressure value, with a consequentreduction in the absorbed power, control unit 62 will generate asuitable command for actuator 50, so as to further reduce thedisplacement.

The rotation of the rings may continue until the position shown in FIG.1 is attained, where centres O and O′ coincide and vanes 16 and centringrings 17 rotate with the rotor without changes in their radial relativeposition. Consequently, the displacement is null and the pump is install condition. It is to be pointed out that this position may be takenwhen a hydraulic lock of the delivery pressure is approaching. In theconstructional practice, a minimum displacement is preferably maintainedby protecting the pump with a maximum pressure valve.

The operation of pump 101 is wholly similar, with the changes due to theprovision of stator ring 42 alone.

An important parameter in managing the delivery rate/pressure of an oilpump for thermal engines is temperature, the increase of which makes oilbecome more fluid and the engine permeability increase. Consequently,the pump displacement should proportionally increase. This may befavoured if the opposing load of the counteracting spring increases. Inorder to obtain this, flat spiral spring 34 may be made of a bimetallicmaterial such that temperature causes an increase in the rigidity andhence in the counteracting torque. In order to obtain the change in therigidity, the small oil flow rate for the lubrication of shaft 54 ofwheel 53 may be exploited: the oil, after having licked casing 33 ofspring 34 and having transmitted its temperature to the same spring,freely discharges towards the suction chamber through the drain providedin chamber 57.

The invention actually attains the desired aims.

Use of an electromagnetic actuator, directly controlled by theelectronic control unit of the vehicle, allows eliminating the hydraulicactuators of the prior art and the necessary connections to thelubrication circuit, and makes the system less cumbersome, simpler andmore reliable as well as less expensive. Moreover, elimination of thehydraulic actuators enables modifying the displacement presetting evenwhen the thermal engine is off, since no control pressure is required.This is advantageous in particular for vehicles provided with the “stopand go” function, because it allows for instance increasing thedisplacement between the stop of the thermal engine and its start inorder to start again the engine with a good lubrication.

Moreover in both embodiments, given the respective rotation direction ofthe rotor, in case of an electric failure causing deactivation ofactuator 50 the hydraulic torque of the pump causes the rotation of thestator ring, or of the stator ring and the external ring, towards themaximum displacement condition. As said, this action can be favoured byspring 34. In case of other failures, the minimum displacement isensured and the electronic control, by detecting low lubricationpressures, will bring control unit 62 to the vehicle “recovery”function,

It is clear that the above description has been given only by way ofnon-limiting example and that changes and modifications are possiblewithout departing from the scope of the invention.

For instance, even if in the illustrated embodiment shaft 15 a of rotor15 is guided by body 10 whereas spiral spring 34 and the calibrationmeans consisting of casing 33 and ring nut 55 are housed within cover14, the arrangement could be reversed, or also the spring and thecalibration means could be housed within body 10.

Moreover, spring 34 could not be a bimetallic spring and, at least inthe embodiments where actuator 50 is a step-by-step motor, the springcould be dispensed with, the only magnetic resistance torque betweensubsequent steps maintaining the position of rings 12, 42 when the motoris not excited.

Lastly, even if the invention has been disclosed in detail withreference to a pump for the lubrication oil of a motor vehicle engine,it may be applied to any positive displacement pump for conveying afluid from a first to a second working environment, in which a deliveryrate reduction as the pump speed increases is convenient.

1. A variable displacement rotary positive displacement pump for fluids,comprising a rotor (15; 115) arranged to rotate within an eccentriccavity (43; 143) of a stator ring (42; 142) in turn arranged to berotated within a predetermined angular interval, as operating conditionsof the pump (1; 101) vary and upon command of a system (62, 63)detecting such conditions, in order to vary a relative eccentricitybetween the eccentric cavity (43; 143) and the rotor (15; 115) and hencethe pump displacement, the pump (1; 101) further including anelectromagnetic actuator (50), integrated into or coupled with the pump,which is driven by said detecting system (62, 63) and is arranged togenerate a rotary motion and to transmit it to said stator ring (42;142) through a toothed wheel (53; 153); the pump being characterised inthat: said toothed wheel (53; 153) is located at least in part in astator cavity (11 a, 11 b; 111) located in a peripheral positionrelative to said stator ring (42; 142); and in that a toothed sector(52; 152) is located at the base of said peripheral stator cavity (11 a,11 b; 111).
 2. The pump as claimed in claim 1, wherein said toothedsector (52; 152) located at the base of said peripheral stator cavity(11 a, 11 b; 111) meshes with said toothed wheel (53; 153) driven by theactuator (50) and develops according to a profile defined by an involuteof the teeth of the wheel (53; 153).
 3. The pump as claimed in claim 2,wherein the stator ring (42) is housed within an eccentric cavity (13)of an external ring (12), and said actuator (50) is arranged to transmitthe rotary motion to both rings (12; 42) in such a way as to cause asynchronous rotation thereof by an equal amount in opposite directions.4. The pump as claimed in claim 3, wherein the eccentric cavities (13,43) have the same eccentricity and, in a minimum displacement condition,are arranged so that their eccentricities are offset by 180°.
 5. Thepump as claimed in claim 3, wherein facing surfaces of the external ring(12) and the stator ring (42) have formed thereon respective toothedsectors (51, 52) with which a toothed wheel (53) driven by the actuator(50) meshes and which develop according to a profile defined by aninvolute of the teeth of the wheel (53), so that, during the rotation ofthe rings (12, 42), a centre (O′) of the cavity of the stator ring movesalong a rectilinear path.
 6. The pump as claimed in claim 2, wherein thetoothed wheel (53; 153) is arranged to cooperate with a member (34)opposing the rotation of the stator ring (42; 142), which memberconsists of a flat spiral spring secured at one end to a shaft (54) ofthe toothed wheel and at the other end to an element (33) rigidlyconnected to the pump body, the spring being associated with settingmeans (33, 55) arranged to set a desired steady state value for thedisplacement of the pump (1; 101), and wherein the flat spiral spring(34) is made of a bimetallic material and has a temperature-dependingcharacteristic.
 7. The pump as claimed in claim 2, wherein the actuator(50) is a step-by-step micromotor or is a linearly moving actuatorequipped with an escapement ratchet gear arranged to convert theactuator motion into a rotary motion.
 8. The pump as claimed in claim 1,wherein the pump (1; 101) is a pump for a lubrication circuit (60) of amotor vehicle engine (61).
 9. A method of regulating the displacement ofa rotary positive displacement pump (1; 101) of a kind comprising arotor (15; 115) arranged to rotate within an eccentric cavity (43; 143)of a stator ring (42; 142), the method comprising the step of making thestator ring (42; 142) rotate within a predetermined angular interval inorder to vary the eccentricity between the cavity (43; 143) and therotor (15; 115) as operating conditions of the pump (1; 101) vary, andbeing characterised in that it further comprises the steps of: providingan electromagnetic actuator (50) integrated into or coupled with thepump and arranged to transmit a rotary motion to the stator ring (42)through a toothed wheel (53; 153) located at least in part in a statorcavity (11 a, 11 b; 111) located in a peripheral position relative tosaid stator ring (42; 142); supplying the actuator (50) with commandscorresponding to a desired rotation of the stator ring (42; 142). 10.The method as claimed in claim 9, further comprising the steps of:providing an external ring (12) having an eccentric cavity (13) withinwhich the stator ring (42) is housed; and making the stator ring (42)and the external ring (12) rotate by a same angle at the same time andin opposite directions.
 11. The method as claimed in claim 9 or 10,arranged for regulating the displacement of a pump for the lubricationoil of a motor vehicle engine.
 12. A lubrication system for a motorvehicle engine (61), comprising a pump (1; 101) as claimed in claim 1.13. The pump as claimed in claim 1, wherein the stator ring (42) ishoused within an eccentric cavity (13) of an external ring (12), andsaid actuator (50) is arranged to transmit the rotary motion to bothrings (12; 42) in such a way as to cause a synchronous rotation thereofby an equal amount in opposite directions.
 14. The pump as claimed inclaim 13, wherein the eccentric cavities (13, 43) have the sameeccentricity and, in a minimum displacement condition, are arranged sothat their eccentricities are offset by 180°.
 15. The pump as claimed inclaim 14, wherein facing surfaces of the external ring (12) and thestator ring (42) have formed thereon respective toothed sectors (51, 52)with which a toothed wheel (53) driven by the actuator (50) meshes andwhich develop according to a profile defined by an involute of the teethof the wheel (53), so that, during the rotation of the rings (12, 42), acentre (O′) of the cavity of the stator ring moves along a rectilinearpath.
 16. The pump as claimed in claim 13, wherein facing surfaces ofthe external ring (12) and the stator ring (42) have formed thereonrespective toothed sectors (51, 52) with which a toothed wheel (53)driven by the actuator (50) meshes and which develop according to aprofile defined by an involute of the teeth of the wheel (53), so that,during the rotation of the rings (12, 42), a centre (O′) of the cavityof the stator ring moves along a rectilinear path.
 17. The pump asclaimed in claim 13, wherein the toothed wheel (53; 153) is arranged tocooperate with a member (34) opposing the rotation of the stator ring(42; 142), which member consists of a flat spiral spring secured at oneend to a shaft (54) of the toothed wheel and at the other end to anelement (33) rigidly connected to the pump body, the spring beingassociated with setting means (33, 55) arranged to set a desired steadystate value for the displacement of the pump (1; 101), and wherein theflat spiral spring (34) is made of a bimetallic material and has atemperature-depending characteristic.
 18. The pump as claimed in claim13, wherein the actuator (50) is a step-by-step micromotor or is alinearly moving actuator equipped with an escapement ratchet geararranged to convert the actuator motion into a rotary motion.
 19. Thepump as claimed in claim 1, wherein the toothed wheel (53; 153) isarranged to cooperate with a member (34) opposing the rotation of thestator ring (42; 142), which member consists of a flat spiral springsecured at one end to a shaft (54) of the toothed wheel and at the otherend to an element (33) rigidly connected to the pump body, the springbeing associated with setting means (33, 55) arranged to set a desiredsteady state value for the displacement of the pump (1; 101), andwherein the flat spiral spring (34) is made of a bimetallic material andhas a temperature-depending characteristic.
 20. The pump as claimed inclaim 1, wherein the actuator (50) is a step-by-step micromotor or is alinearly moving actuator equipped with an escapement ratchet geararranged to convert the actuator motion into a rotary motion.